This invention relates generally to turbine components and more particularly to a combustion chamber.
Industrial gas turbine combustors are typically designed as a plurality of discrete combustion chambers or “cans” in an array around the circumference of the turbine. Conventionally, the walls of an industrial gas turbine can combustion chamber are formed from two major pieces: a cylindrical or cone-shaped sheet metal liner engaging the round head end and a sheet metal transition piece that transitions the hot gas flowpath from the round cross-section of the liner to an arc-shaped sector of the inlet to the turbine. These two pieces are mated with a flexible joint, which requires some portion of compressor discharge air to be consumed in cooling flow and leakage at the joint.
Traditional gas turbine combustors use diffusion (i.e., non-premixed) combustion in which fuel and air enter the combustion chamber separately. The process of mixing and burning produces flame temperatures exceeding 3900° F. Since conventional combustor liners and/or transition pieces having metallic walls are generally capable of withstanding a maximum metal temperature on the order of only about 1500° F. for about ten thousand hours (10,000 hrs.), steps to protect the combustor liner and/or transition piece must be taken.
Because diatomic nitrogen rapidly dissociates at temperatures exceeding about 3000° F. (about 1650° C.), the high temperatures of diffusion combustion result in relatively high NOx emissions. One approach to reducing NOx emissions has been to premix the maximum possible amount of compressor air with fuel. The resulting lean premixed combustion produces cooler flame temperatures and thus lower NOx emissions. The assignee of the present invention has used the term “Dry Low NOx” (DLN) to refer to lean premixed combustion systems with no diluents (e.g., water injection) for further flame temperature reduction. Although lean premixed combustion is cooler than diffusion combustion, the flame temperature is still too hot for uncooled combustor components to withstand.
Furthermore, because the advanced combustors premix the maximum possible amount of air with the fuel for NOx reduction, little or no cooling air is available, making film-cooling of the combustor liner and transition piece impractical. Nevertheless, combustor chamber walls require active cooling to maintain material temperatures below limits. In DLN combustion systems, this cooling can only be supplied as cold side convection. Such cooling must be performed within the requirements of thermal gradients and pressure loss. Thus, means such as thermal barrier coatings in conjunction with “backside” cooling have been considered to protect the combustor liner and transition piece from destruction by such high heat. Backside cooling involves passing the compressor discharge air over the outer surface of the transition piece and combustor liner prior to premixing the air with the fuel.
At temperatures consistent with current-technology DLN combustion, some enhancement of backside convective heat transfer is needed, over and above the heat transfer that can be achieved with simple convective cooling and within acceptable pressure losses. With respect to the combustor liner, one current practice is to impingement cool the liner. Another practice is to provide linear turbulators on the exterior surface of the liner. Another more recent practice is to provide an array of concavities on the exterior or outside surface of the liner (see U.S. Pat. No. 6,098,397). The various known techniques enhance heat transfer but with varying effects on thermal gradients and pressure losses. Turbulation strips work by providing a blunt body in the flow which disrupts the flow creating shear layers and high turbulence to enhance heat transfer on the surface. Dimple concavities function by providing organized vortices that enhance flow mixing and scrub the surface to improve heat transfer.
A low heat transfer rate from the cold side of the liner can lead to high liner surface temperatures and ultimately loss of strength. Several potential failure modes due to the high temperature of the liner include, but are not limited to, cracking, bulging and oxidation. These mechanisms shorten the life of the liner, requiring replacement of the part prematurely.
Additionally, conventional can combustors present a long flow path to the system, resulting in high pressure loss and long residence time of the hot gas. Long residence time is beneficial to CO reduction at low power, low temperature conditions, but is detrimental to NOx formation at high power, high temperature conditions.
Accordingly, there remains a need for a combustor that completes combustion with low emissions and low pressure loss, that presents sufficient residence time to the hot gas to complete the combustion process without excessive CO formation, and that allows for adequate mixing of the burned gases to reduce the temperature non-uniformity entering the turbine, and that preserves the maximum possible amount of compressor discharge air for premixing.